Pre/Post Development Comparison

One watershed for each condition

Land-use pieces are weighted by area into one curve number or one C, as TR-55 Worksheet 2 and FHWA HEC-22 Eq. 4.2 do. They are not separate subbasins: nothing is routed, and peaks are not added.

NRCS mode

For each return period the 24-hour depth gives runoff by TR-55 Eqs. 2-1 to 2-4 and the peak by the Graphical Peak Discharge method (Chapter 4), for each condition with its own CN and Tc. Volume is runoff depth times area.

Storage estimate

With a target release, V_s = V_r (V_s/V_r) with V_s/V_r from q_o/q_i (TR-55 Chapter 6, Eqs. 6-1 and 6-2). Figure 6-1 is a curve in TR-55; this page uses the polynomial FHWA HEC-22 4th ed. publishes for it (Eq. 10.5, Table 10.1), only inside the figure’s range, q_o/q_i from 0.1 to 0.8.

Rational mode

Q = C i A for each condition, with the intensity for its own Tc. The method gives a peak, not a runoff volume, so this mode has no volume or storage columns.

Numbers from a report

Paste the flows of a drainage report by outlet and event, with the flow after detention and a target if you have them. The page works out the differences and does not recompute any flow.

Target release

You enter the target, or choose the pre-development flow of each event as a starting point. It is never applied unless you choose it. The results are numbers side by side.

Rainfall from NOAA Atlas 14

For a Texas location, 24-hour depths or intensities come from NOAA Atlas 14 Volume 11 for the 2- to 100-year return periods, in the partial-duration or annual maximum series. Or type your own.

The calculator is tested against the cases below every time its code changes.

The storage estimate is compared with NRCS TR-55 Examples 6-1 and 6-2 and FHWA HEC-22 Example 10.2 to the precision they print: TR-55 reads V_s/V_r from a curve to two decimals, and HEC-22 rounds it before multiplying, so that case has 2%. The comparison itself is checked by hand calculations of the published equations. No reference program has been run for this page.

• A comparison and a preliminary estimate: no hydrograph and no routing through a pond. TR-55 says not to use its storage estimate for final design if a 25 percent storage error cannot be tolerated, and that it can overestimate storage. A routing check is still needed: Detention Routing does that.
• Each condition is one homogeneous watershed with one outlet. Several subbasins, flows from outside the site, and the timing of peaks at a downstream point are not computed; compare them with a hydrograph program and bring the results in as report numbers.
• The graphical peak needs Tc from 0.1 to 10 hours, a weighted CN over 40 and TR-55 Type II or III rainfall. Antecedent runoff condition II and I_a = 0.2S only. Which rainfall distribution applies in your area is for your reviewing agency to set.
• TR-55 also estimates the outflow for a given storage (Worksheet 6b) and two-stage structures. This page computes storage for a target only, event by event.
• Nothing here states what your criteria require. Which return periods, target release and method apply come from your reviewing agency. See the Texas Drainage Criteria Directory for the published documents.

Can it check the numbers in a drainage report?

Yes, as arithmetic. Choose Numbers from a report and paste one row per outlet and event: pre, post, after detention and target if you have them. It lines them up and shows the differences. To test the flows themselves, recompute them from the inputs in NRCS or Rational mode.

Why does Rational mode show no volume?

The Rational Method estimates a peak, not a hydrograph. A volume would need a shape assumed for the hydrograph, and that is a different method. Use NRCS mode or a hydrograph program for volume and storage.

Is the storage estimate a pond size?

No. It is the TR-55 Chapter 6 preliminary estimate, from average routing effects. A pond needs a stage-storage curve, an outlet and a routing of the inflow hydrograph.

Should the target be the pre-development peak?

Often that is the starting point, but your reviewing agency sets the release, sometimes for several return periods and sometimes as a fixed rate. Enter what its criteria say.

Can I attach the calculation sheet to a report?

You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews and is responsible for what goes into a report.